­­­­­­­­­KINEMATIC HISTORY OF DUCTILE AND BRITTLE DEFORMATION AT THE BLUE RIDGE-PIEDMONT BOUNDARY, CENTRAL VIRGINIA

The boundary between the Blue Ridge and Piedmont geologic provinces in the central and southern Appalachians has long been inferred to be a major tectonic boundary, but in central Virginia the nature of this structure is poorly understood. New geologic mapping and structural analysis in the Howardsville 7.5’ quadrangle provides critical data regarding the kinematic history of this boundary. The Eastern Blue Ridge (EBR) consists of a thick (>4 km) Neoproteorozoic to Early Paleozoic cover sequence of metamorphosed sedimentary and volcanic units (Lynchburg Group, Catoctin Formation, and Evington Group). Rocks in the EBR preserve a NE-striking, SE-dipping greenschist-facies foliation that formed during NW-directed contractional deformation. The western Piedmont (WP) includes a thick sequence of phyllite interlayered with quartzose to feldspathic metawackes. Foliation in the WP is typically shallowly dipping and shear bands are common.

In the Howardsville area, a ~5-km wide transition zone of steeply dipping and strongly deformed rocks separates the EBR and WP. Strain and kinematic indicators record strike-parallel (NE/SW) elongation, and general shear with an overall dextral transpressional geometry. Two Triassic half-grabens (Scottsville and Midway Mills basins) cut the Paleozoic structures, and are bounded to the west by NE-striking, SE-dipping normal faults. A suite of N- to NNW-striking Jurassic dikes intrudes older rocks and structures.

Rocks in both the EBR and WP are cut by a steeply dipping, WNW- to NW-striking joint set absent in the Mesozoic rocks. This major fracture set formed with the maximum principal stress oriented NW-SE, and likely developed during late Alleghanian (~280 to 300 Ma) regional shortening. The youngest brittle deformation features occur as sub-vertical joints in Mesozoic rocks, and include a NE-striking set cut by a NNW-striking set. The NNW-striking set is parallel to Jurassic diabase dikes, likely requiring a 30˚ to 40˚ rotation of the principal stress direction between the late Triassic and Jurassic.